DE102010001808A1 - wastewater treatment plant - Google Patents

Abstract

The invention relates to a wastewater treatment plant for the purification of wastewater (1) which is obtained in a dewatering device (7) for dewatering a fiber suspension (2) suitable for producing a paper, cardboard, tissue or other fibrous web. The load on the water cycle during the production of the fibrous suspension (2) and the fibrous web with colloidally dissolved substances is to be reduced in an efficient manner by at least part of the wastewater (1) from the dewatering device (7) in an electrophysical cleaning stage (5) Coagulation and flocculation and then into a separation stage (6) with flake enlargement and flake separation.

Description

The invention relates to a wastewater treatment plant for the purification of wastewater, which is obtained in a dewatering apparatus for dewatering a suitable for producing a paper, cardboard, tissue or other fibrous web pulp suspension.

The increasing use of rejects and waste paper in papermaking and the increased reduction in the use of fresh water have led to an increase in harmful or disturbing substances in the water cycles. Also, chemical additives (eg oils, solvents, resin glues, synthetic sizing agents, adhesives, wet strength agents, retention agents, starch, biocide formulations, dispersants, bleach chemicals, cleaning agents, dyes, complexing agents and solubilizers) which are supplied to the process in a targeted manner are supported by enrichment the circuits contribute to an increase in the concentration of colloidally dissolved impurities directly or from the interaction with each other. Other sources include extracts from the pulps, lignin and lignin derivatives, hemicelluloses and carbohydrates.

The growing concentration of contaminants leads to a reduced efficiency of the mostly cationic functional chemicals (eg fixatives, retention polymers). The saturation of the process water with colloidally dissolved, anionic impurities, which occurs at high process temperatures, leads to precipitations and deposits in the cooler zones. Even low temperature gradients are sufficient to cause sticky deposits on hydrophobic or particularly adhesive surfaces (screen material, felt material, roll surfaces, low-flow zones). These can affect the process sensitively by the formation of holes in the paper, breaks, cleaning stops.

Paper properties such as whiteness, opacity, color and strength are also affected by the presence of colloidal impurities. In addition, an increased tendency to odor in the paper may occur.

The impurities can also lead to increased foam by lowering the surface tension, which has a negative impact on the paper quality or makes the increased use of antifoaming agents required.

The accumulation of contaminants in the entire water cycle system depends on the amount of raw materials supplied, the process temperature, the extractability, the water circulation rate, the amount discharged with the wastewater, the discharge of impurities with the paper produced and the supply of fresh water.

In particular for the narrowing of the water cycles, d. H. the reduced supply of fresh water and the correspondingly reduced removal of wastewater sets up an increased concentration of colloidally dissolved impurities in the circulating waters. In addition to the high concentration level, dynamic fluctuations of the contaminant loads also represent a limitation for a targeted chemical-technological management of the process. This leads to continuous incorrect dosing of functional chemicals with the effects described above.

The object of the invention is therefore to enable the removal of colloidally dissolved impurities in an efficient manner.

According to the invention, this object is achieved in that at least a portion of the wastewater of the dewatering apparatus is passed into an electrophysical cleaning stage for coagulation and flocculation and then into a separation stage with flake enlargement and flake separation.

By focusing on the treatment of the wastewater of the dewatering device, the cost of the electro-physical cleaning can be kept within reasonable limits. Nevertheless, this cleaning causes a significant minimization of the burden of water cycles in the production of the pulp suspension and thus also in the machine for producing the fibrous web.

Furthermore, the electrophysical cleaning of the effluent of the dewatering apparatus is very efficient in reducing concentration peaks in terms of loading with colloidally dissolved contaminants in the circuits.

In this case, the dewatering apparatus is generally formed by a disk filter, a cloth press or the like. A combination of different drainage devices may also be advantageous.

To minimize the fiber losses during drainage, it is advantageous if the wastewater before the electrophysical purification stage by a Entstoffungsvorrichtung, preferably a filter is performed. The substance filtered out in the process is essentially formed by fibers and fillers and should preferably be returned to the pulp suspension before the dewatering device.

As a result, at least a portion of the effluent purified in the electrophysical purification stage may be recycled as a partial or complete fresh water replacement to the fibrous suspension or fibrous web manufacturing process.

In the preparation of the pulp suspension, the purified wastewater can be mixed in the stock preparation or in the constant part of the pulp suspension. In stock preparation, a pulp suspension of pulp and / or waste paper is made with the addition of water and additives. In the constant part, the highly consistent pulp suspension of the stock preparation, which is essentially formed of processed fibers and fillers, is diluted with water and then fed to the headbox of the production machine.

It is advantageous if the purified wastewater is returned after the separation stage in a range of stock preparation for pulp suspension, which is located in front of the dewatering device.

Often, the need and / or the accumulation of purified wastewater fluctuates, which is why the purified wastewater should be directed to the buffering before returning to the pulp suspension in a collection container.

Depending on the load of the wastewater, it may be necessary for all the wastewater of the dewatering device to be led into the electrophysical purification stage. Often, however, it is sufficient if only a part of the waste water of the dewatering apparatus is guided into the electrophysical cleaning stage.

It is also advantageous if, prior to the electrophysical cleaning stage, preferably between the Entstoffungsvorrichtung and the electrophysical cleaning stage, a cleaning stage with wet oxygenation is present.

Wet oxidation is understood to mean the chemical oxidation of organic compounds in a liquid phase.

In this case, the wet oxidation by the introduction of ozone or oxygen (at high pressure and high temperature) can be brought about. However, it is considerably more efficient and less costly if the wet-oxidation purification stage is designed to be electrochemically and / or UV-supported.

In electrochemical wet oxidation, radicals or strong oxidants (ozone, OH ^- , O ⁺ ) are produced by the decomposition of water by electrolytic means.

By applying a DC voltage, the water is decomposed, producing at the anode OH radicals and above a certain current ozone and at the cathode hydrogen.

For this purpose, inert electrodes, preferably diamond electrodes should be used.

In UV-based wet oxidation, the radicals or oxidants are generated by the irradiation of the waste water with UV radiation.

To intensify the COD reduction, it may be advantageous to use the electrochemical and the UV-supported wet oxidation in combination.

As a result, in particular, colloidally dissolved impurities in the wastewater are oxidized by this wet oxydation and their long-chain molecules are broken up. The broken-up molecules can then be broken down in the subsequent purification stage.

In the purification step with electrophysical precipitation for coagulation and flocculation of colloidally dissolved impurities, the water to be treated is passed through a reactor equipped with sacrificial electrodes in which various electrochemical reactions take place by applying an electric current.

This produces metal hydroxide flakes and various highly reactive radicals. The metal hydroxide flakes have a high adsorption capacity and can thus bind finely distributed particles to themselves. It also leads to mit precipitation and inclusion precipitation reactions in which the colloidally dissolved substances are precipitated.

In addition to the precipitation reactions, there are also oxidative effects, mainly by highly reactive oxygen radicals, which leads to further degradation of organic compounds.

In the subsequent to the electrophysical purification stage deposition stage it comes to flake enlargement and flocculation, in particular by means of flotation, sedimentation, filtration o. Ä. Accordingly, the particle agglomerates, which were generated by means of electro-physical precipitation / flocculation in the previous step, here from purified filtrate separated.

In particular, the electrophysical cleaning stage allows in the invention demand-oriented, targeted transfer and discharge of the mostly anionic contaminant as a solid.

With regard to the control / regulation of the sewage treatment plant, it is advantageous if the load level of the wastewater is measured and the intensity of the electro-physical cleaning is at least partially controlled / regulated in a manner that balances the stress level over time.

On the one hand, this involves reducing the overall load level of the waters with colloidally dissolved impurities, so that certain water qualities are achieved, which reduce the use of fresh water.

Another often much more important object of the invention is the timely elimination or reduction of peak loads with the aim of setting a constant Störfrachtniveaus.

The measurement of the load level of the waste water, d. H. Concentration of impurities should be determined by measuring chemical oxygen demand (COD), dissolved carbon (DOC), turbidity, anionic charge character (SCD), evaporation residue, titrated cationic demand or similar.

In this case, the intensity of the electro-physical cleaning on the electrical power consumption, d. H. be controlled / regulated by the amount of voltage applied to the electrodes and / or the current.

The invention should advantageously be used in particular in partially or completely closed processes with high levels of contaminant load and / or large fluctuations and / or small amounts of sewage or fresh water, since in these cases the effects of the concentration of colloidally dissolved impurities on the process stability are particularly disturbing are.

The invention will be explained in more detail with reference to an exemplary embodiment. In the accompanying drawing, the figure shows an investment scheme of a wastewater treatment plant for stock preparation 8th ,

The required for the production of a fibrous web pulp suspension 2 is used in stock preparation 8th made of pulp and / or waste paper with the addition of water, wherein the pulp suspension 2 in the stock preparation 8th several stages of treatment, such as bleaching, grinding, etc. goes through.

After the stock preparation 8th the pulp suspension arrives 2 in the constant part 9 , in which additives are added and the consistency of the pulp suspension 2 changed so, is usually lowered by dilution, that this to the headbox of the subsequent paper machine 10 can be performed. Thinning in the constant part 9 is mostly with the white water from the paper machine 10 realized.

The paper machine 10 is used to produce a fibrous web concretely a paper web and starts with a headbox, the pulp suspension 2 onto an endlessly circulating sieve or between two surrounding sieves of a leaf forming area.

In the leaf forming area, a considerable amount of water is separated via the sieves and collected as white water in the white water tank. It comes to sheet formation, which allows the subsequent dewatering of the fibrous web in a press section.

In the press section, the fibrous web is guided together with at least one water-receiving, endless circulating belt through at least one press nip. The press water pressed out of the fibrous web is likewise guided into a white water container and reused.

After the press section, the fibrous web passes through several treatment stages, such as a drying section for drying the fibrous web, possibly also a coating device and usually a smoothing device for smoothing the fibrous web before it can then be wound up.

In the stock preparation 8th As a rule, there is also a thickening of the pulp suspension 2 by means of at least one dewatering device 7 What the further treatment of the pulp suspension 2 makes it more efficient.

As a dewatering device 7 For the most part, disc filters or cloth presses are used.

The thickening of the pulp suspension 2 accumulating wastewater 1 is through a Entstoffungsvorrichtung 3 passed in the form of a filter, in the, essentially of fibers and fillers formed substance 12 from the sewage 1 filtered out and reused in the stock preparation 8th is returned. This recycling is advantageously carried out in front of the dewatering device 7 lying area of stock preparation 8th ,

Because the sewage 1 even after the de-staining device 3 still has a relatively high proportion of colloidally dissolved impurities, this wastewater 1 subsequently subjected to a special treatment.

In this way, peak loads can be reduced and because of the largely closed water cycle also generally the amount of stress can be substantially reduced.

The entire wastewater 1 the dedusting device 3 this is done in an electrophysical cleaning stage 5 led to coagulation and flocculation.

This is possible because the sewage 1 because of the dedusting device 3 already sufficiently freed of suspended solids.

In the interest of maximum reduction of COD, the wastewater 1 as shown in the figure, from the dedusting device 3 via a cleaning stage 4 with wet oxidation for the electrophysical cleaning stage 5 be directed.

Easily oxidizable substances are to be converted into anionically charged impurities, the precipitation and flocculation treatment in an electrophysical purification stage 5 are more accessible.

With the help of inert electrodes and an applied electrical potential difference are in the purification stage 4 Produced with wet oxydation ozone and highly reactive OH ^- radicals, which act as an oxidizing agent. In the course of this process, colloidally dissolved impurities are oxidized in the water.

Disturbances that consist of long-chain molecules are broken during this oxidation process, and thus made accessible to the downstream electrophysical precipitation / flocculation.

After the optional wet oxidation 4 goes through the sewage 1 a cleaning step 5 for coagulation and flocculation.

In this case, the colloidally dissolved impurities (anionically charged interfering substances, such as, for example: hemicelluloses, resins, polysaccharides, etc., which occur in particular in wastewaters of papermaking) are to be converted into larger agglomerates.

For this purpose, the wastewater 1 led into a container which has a sacrificial anode, for example made of aluminum and a particular inert cathode, which are connected via a control unit with a DC voltage source. However, the cathode can also consist of sacrificial anode material, which offers the possibility of pole reversal.

As already described, the electric current flow leads to an electrophysical precipitation for the purpose of coagulation and flocculation of colloidally dissolved impurities.

Depending on the measured COD value of the wastewater 1 can the current through the control unit be changed so that it comes not only to a reduction but also in terms of time to a homogenization of the residual COD.

In the refer to the electrophysical cleaning stage 5 subsequent deposition stage 6 it comes to flake enlargement and flocculation, in particular by means of flotation, sedimentation, filtration o. Ä.

In this case, the particle agglomerates, which by means of the electro-physical cleaning stage 5 were generated by the sewage 1 separated and discharged.

The cleaned wastewater 1 is then in a collection container 11 led, from where it into the stock preparation 8th , preferably in the front of the dewatering device 7 lying area is passed.

The in the separation stage 6 accumulating precipitate 13 is generally fed to the sludge treatment.

By the invention, the proportion of continuously added fresh water as well as the discharged amount of waste water 1 significantly reduce.

Claims (12)

Wastewater treatment plant for the purification of wastewater ( 1 ), which in a dewatering device ( 7 ) for dewatering a fibrous suspension suitable for producing a paper, board, tissue or other fibrous web ( 2 ), characterized in that at least part of the wastewater ( 1 ) of the dewatering device ( 7 ) in an electrophysical purification stage ( 5 ) for coagulation and flocculation and then in a deposition stage ( 6 ) with flake enlargement and flake separation. Wastewater treatment plant according to claim 1, characterized in that the wastewater ( 1 ) before the electrophysical cleaning step ( 5 ) by a degassing device ( 3 ), in particular a filter is performed. Wastewater treatment plant according to claim 2, characterized in that by the Entstoffungsvorrichtung ( 3 ) from the wastewater ( 1 ) removed substance ( 12 ) preferably in front of the dewatering device ( 7 ) to the pulp suspension ( 2 ) is returned. Wastewater treatment plant according to one of the preceding claims, characterized in that the purified wastewater ( 1 ) after the separation stage ( 6 ) preferably in front of the dewatering device ( 7 ) to the pulp suspension ( 2 ) is returned. Wastewater treatment plant according to claim 4, characterized in that the purified wastewater ( 1 ) before returning to the pulp suspension ( 2 ) in a collecting container ( 11 ). Wastewater treatment plant according to one of the preceding claims, characterized in that the entire wastewater ( 1 ) of the dewatering device ( 7 ) into the electrophysical purification stage ( 5 ) to be led. Wastewater treatment plant according to one of claims 1 to 5, characterized in that only a part of the waste water ( 1 ) of the dewatering device ( 7 ) into the electrophysical purification stage ( 5 ) to be led. Wastewater treatment plant according to one of the preceding claims, characterized in that the dewatering device ( 7 ) is formed by a disc filter or a cloth press. Wastewater treatment plant according to one of the preceding claims, characterized in that prior to the electrophysical purification stage ( 5 ), preferably between the degassing device ( 3 ) and the electrophysical purification stage ( 5 ) a cleaning step ( 4 ) is present with wet oxydation. Method for controlling the wastewater treatment plant according to one of the preceding claims, characterized in that the load level of the wastewater ( 1 ) and the intensity of the electrophysical cleaning is at least partially controlled in a manner that balances the stress level over time. Method for controlling the wastewater treatment plant according to claim 10, characterized in that the load level of the wastewater ( 1 ) is determined by measuring the chemical oxygen demand (COD), the dissolved carbon (DOC), the turbidity, the anionic charge character (SCD), the evaporation residue, the titrated cationic demand or similar. Method for controlling the wastewater treatment plant according to claim 10 or 11, characterized in that the intensity of the electro-physical cleaning is controlled by the electrical power consumption.

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